1. Field of the Invention
The present invention relates to a fault determination apparatus and fault determination method for a battery set and more particularly to a fault determination apparatus and fault determination method for a battery set formed by a plurality of serially connected cells.
2. Description of the Related Art
One fault determination apparatus proposed for this type of battery set determines fault on the basis of the magnitude of internal-resistance calculated on each cell, or on each block including a plurality of cells. Because the internal resistance of a cell depends on its construction, the fault of the cell can be determined by calculating the internal resistance and using the resulting value as determination criteria.
However, such apparatuses which determine fault by calculating the internal resistance and using the resulting value as criteria suffer in that certain charge-and-discharge patterns lead to difficulties in accurate determination. Electromotive force of the battery set varies according to the charge-and-discharge patterns. Even though the internal resistance can be accurately calculated when the electromotive force of the battery does not vary, the internal resistance can assume a normal or an abnormal value depending on the charge-and-discharge pattern. The fact that the electromotive force of the battery also varies as the temperature of the battery changes exacerbates the problem outlined above. In addition, calculating the internal resistance for each of the cells or each block requires such an enormous amount of computing processes that a high-performance computer must be used for the computation.
It is an object of the present invention to provide a fault determination apparatus and fault determination method for a battery set, which do not suffer from the above problems and which can accurately determine fault of a battery set regardless of variations in the charge-and-discharge patterns or in temperature; it is a further object of the present invention to provide a fault determination apparatus and fault determination method for a battery set capable of executing said fault determination with simple computation.
To achieve the above objects to a varying degree, the fault determination apparatus and fault determination method for a battery set of the present invention can be configured as described below.
The fault determination apparatus for a battery set of the present invention for determining fault of a battery set comprising a plurality of serially connected cells, comprises a voltmeter for measuring the voltage of each of the plurality of cells, an ammeter for measuring electric current passing through said battery set, and a fault determination means for receiving input values of said voltage measured by said voltmeter at predetermined timing, computing a voltage difference between the maximum value and the minimum value of said voltage, inputting the value of electric current measured by said ammeter at the same timing with said predetermined timing, storing a plurality of value pairs containing said electric current and said computed voltage difference, and determining fault of said battery set on the basis of the stored value pair.
The principle of fault determination of a battery set through use of a plurality of value pairs containing difference between the maximum and the minimum voltage values and the current passing through a battery set is described below. Voltage (V) of a battery can be represented by subtracting the product of current (I) passing through the battery and the internal resistance (R) from the electromotive force (V0) as shown in the following equation (1).
V=V0xe2x88x92Rxc2x7Ixe2x80x83xe2x80x83(1)
In a battery set in which normal and abnormal cells are connected in series to each other, voltages of both normal and abnormal cells can be represented by the above equation (1) and the following equations (2) and (3) can be obtained. In these equations, both currents (I) passing through the cells are equal because the battery set is formed by normal cells and abnormal cells connected in series.
V1=V01xe2x88x92R1xc2x7Ixe2x80x83xe2x80x83(2)
V2=V02xe2x88x92R2xc2x7Ixe2x80x83xe2x80x83(3)
By subtracting equation (2) from equation (3), the following equation (4) can be obtained, wherein the left side of the equation represents the voltage difference (xcex94V).
xcex94V=V2xe2x88x92V1=(V02xe2x88x92V01)xe2x88x92(R2xe2x88x92R1)xc2x7Ixe2x80x83xe2x80x83(4)
When the voltage of each of the cells and the current of the battery set can be measured, the voltage difference (xcex94V) and the current (I) can be treated as known values, and therefore, only the values of the electromotive-force difference and the internal-resistance difference are remain unknown. The electromotive-force difference and the internal-resistance difference can be calculated from two or more value pairs containing voltage difference and current. Because battery fault usually manifests itself in the internal-resistance difference, fault can be determined from the internal-resistance difference. In other words, when one of the cells is normal, the normality of the other cell can be determined on the basis of a plurality of the value pairs, by finding a value corresponding to the internal-resistance difference from the plurality of the value pairs.
Cell fault is determined by assuming the other cell to be normal in the principle described above. In the fault determination apparatus for a battery set of the present invention, fault of a battery set is determined by first computing the voltage difference between the maximum and the minimum voltage values of each of the cells, assuming that the cell with the maximum voltage value is normal and that the cell with the minimum voltage value can be a cell with a fault, and determining fault of the battery set by examining the cell having the minimum voltage value.
The fault determination apparatus for a battery set of the present invention allows determination of battery set fault on the basis of the voltage difference between the maximum and minimum voltage values of each of the cells in conjunction with the current passing through the battery set. Moreover, because variations in electromotive force are canceled by computing a difference, the fault determination is not affected by the charge-and-discharge patterns and temperature fluctuations.
In accordance with one aspect of the fault determination apparatus for a battery set of the present invention, the fault determination means mentioned above can determine the fault when a predetermined number of said value pairs are stored or when said stored current values contained in the plurality of the value pairs are distributed beyond a predetermined range. The fault determination of a battery set becomes more accurate by determining the fault using the predetermined number of value pairs or the distribution beyond the predetermined range as described above.
In addition, in accordance with another aspect of the fault determination apparatus for a battery set of the present invention, said fault determination means can also determine fault by computing the slope using the least-squares method from said plurality of the value pairs, and using said slope for fault determination. The aforementioned equation (4) represents a straight line with the slope being the internal-resistance difference. By calculating the slope with the least-squares method as a linear approximation, fault of the battery set can be determined. In such a fault determination apparatus for a battery set which performs fault determination using slope described above, said fault determination means can also determine that fault is encountered when an absolute value of said slope is larger than a predetermined value.
A fault determination method for a battery set of the present invention for determining fault of a battery set comprising a plurality of serially connected cells, comprises the steps of measuring voltage of each of the plurality of cells at a predetermined timing, computing a voltage difference between the maximum value and the minimum value of the measured voltage, measuring electric current passing through the battery set at the same timing with said predetermined timing, storing a plurality of value pairs containing the measured electric current and said computed voltage difference, and determining fault of said battery set on the basis of the stored value pairs.
In accordance with one aspect of the fault determination method for a battery set of the present invention, a fault can be determined when a predetermined number of said value pairs are stored or when said stored current values contained in the plurality of the value pairs are distributed beyond a predetermined range. In addition, fault can be determined by computing the slope by least-squares method using said plurality of the value pairs values and deciding that fault is encountered when an absolute value of said slope is larger than a predetermined value.